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Defense and Attack Strategies and Mechanisms in Biology 357

A discharge from an electric eel can kill the small fish that are its primary food, but electric eels
can also shock potential predators. A touch from the electric eel’s tail can effectively disable a
human or a large animal with a stunning shock, although a single discharge is usually not enough to
kill. However, repeated shocks could kill.

13.8 ENTANGLERS

13.8.1 Bola

Device consisting of two or three heavy balls attached by one or two ropes or cords and used for
entanglement purposes. It is twirled overhead in one hand and hurled or cast at the intended target.
Designed to entangle legs to retard or stop movement. Probably an ancient weapon, but made
famous by the gauchos of South America, who used them to catch cattle and ostriches (Alexander
et al., 1996).
Ordgarius magnificus, the Australian bola spider, hides in a silk-lined retreat among the
leaves of native trees such as eucalypts. At night it hangs, head down, from a horizontal silk
strand, and using an extended front leg, suspends a silk thread about 4 cm long with a sticky blob on
the end (Figure 13.6). Thread þ blob ¼ bola. The blob contains an attractant moth pheromone.
When the spider detects the vibrations in the air made by an attracted moth flying close, it begins to
jerk its body so as to swing the bola around in a circle. When the moth is close enough, she lets
the thread run then flicks it to hit the moth. The moth is then entangled, the spider reels it in, wraps
it in silk and sucks it dry. Different pheromones are used for different seasons or growth stages
to capture the moth species that are available or are of best size. The difference is that whilst
in technology the target is probably running away, in nature it is flying towards you, with
friendly intent!

Figure 13.6 A bola spider (an American species, Mastophora, is shown here), waiting for a prey insect to fly past.